as your first post, please read the most recent forum rules. So far, no URL / highlighted screenshot posted with your request.

The most generated errors being found on Opencart forum originates from contributed programming. The increased post counters are caused by redundancies of the same solutions that were already provided prior.

Well, it heavy depends on the Theme used, but basically, it's the
.../whatevertheme/template/common/header.tpl
file, where your Link Code has to be added. Below some samples, where such has to be done:
---
Case 1:

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